This invention relates to a closed cavity piston for a hydrostatic power unit. More specifically and without limitation, this invention relates to a new method of manufacturing a closed cavity piston by using metal injection molding techniques to form the body of the closed cavity piston and then sinterbonding an end cap on the piston body.
A closed cavity piston is a piston that includes one or more sealed hollow compartments within the piston. The reduced piston weight provided by the hollow compartments allow for greater hydrostatic shaft speeds, whereas the closed piston end reduces the amount of compressed oil volume in the hydrostatic unit and improves volumetric efficiency and control of swash plate moments. The piston includes a central tube for lubrication and an internal sphere that contacts an external sphere of a mating slipper. The piston also has either a deformable region near the piston sphere for joining the piston onto the slipper, or a deformable region near the slipper sphere for retaining the slipper to the piston depending on whether the piston is male or female.
There are several methods for manufacturing closed cavity pistons for hydrostatic units, but these methods have several cost and technical disadvantages. One method consists of positioning a hollow tube within a hollow piston body and attaching the tube to the piston body with one or more washers that extend radially between the outer wall of the tube and the inner wall of the piston body. The steps of this method include machining bar stock to create a one-piece hollow piston with integral stem, and then securing the stem with a washer. Next, a welder inertia welds a round slug onto the end of the piston. Finally one drills a hole through the stem over the length of the piston. This manufacturing method is very expensive and not practical for pistons with small diameters.
Another method is described in U.S. Pat. No. 6,314,864 and it consists of joining a piston cap to a piston body by welding or brazing. Compared to the method previously described, this method is less expensive and can be applied to small diameter pistons. Still, the manufacturing equipment required to join the cap and body is relatively expensive and the joining process requires an additional manufacturing step subsequent to the metal injection molding process. Furthermore, the joining method introduces additional radial alignment between the piston socket center and the piston body center that could interfere with methods used to join the piston onto the mating slipper.
Because of recent improvements in metal injection molding techniques, making hydraulic components with these techniques has become feasible. The basic process of metal injection molding is that a desirec shape may be created by carving the shape into two blocks of mold. This forms the outer surface of the product that is to be produced. Metallic mold inserts that may be repeatedly used form the interior of the product. Metal powder and non-metallic binding material are then injected into the mold and heated to form the shape dictated by the mold and the insert. After removing the product from the mold, debinding of the product occurs. The debinding process separates the metallic particles from the non-metal particles. After debinding, the product is sintered. Sintering is a process by which the metallic powder may be formed into a coherent mass without melting the material. The same sintering process may be used to join together two or more components in contact with each other. This joining method, or “sinterbonding” process, is superior to conventional methods for combining metal injection molded components because other methods result in poor bond strength. The present invention uses metal injection molded components that are sinterbonded to produce a new closed cavity piston.
Thus, it is a primary object of the present invention to provide a new closed cavity piston that improves upon the state of the art.
Another object of the present invention is to minimize the amount of hydraulic fluid that flows through a closed cavity piston body.
Yet another object of the present invention is to optimize surface geometries between the closed cavity body and end cap to aid sinterbonding the two together.
Another object of the present invention is to provide a magnetic end cap that can capture magnetic particles present in a hydrostatic unit.
Yet another object of the present invention is to use a metal injection molding process and sintering to improve upon the state of the art.
Another object of the present invention is to provide a method of manufacturing a closed cavity piston that is inexpensive.
Yet another object of the present invention is to provide a method of manufacturing a closed cavity piston that can be made with great speed and efficiency.
Another object of the present invention is to provide a method of manufacturing a closed cavity piston using metal injecting molding to form all of the components.
Yet another object of the present invention is to provide a method for sinterbonding metal injected molding objects together to form a closed cavity piston.
Another object of the present invention is to provide a method for manufacturing a closed-cavity female piston that minimizes radial misalignment between the piston socket center and the center of the main piston body.
Yet another object of the present invention is to provide a joining technique that requires, at most, modification of the processes already employed in the metal injection molding process.
These and other objects, features, or advantages of the present invention will become apparent from the specification and claims.